President Reagan was being briefed in the Oval Office for an interview with the television anchormen about his State of the Union address when communications director Patrick J. Buchanan burst in. "Mr. President," Buchanan said, "the shuttle's blown up!"

"That's the one the teacher's on?" Reagan asked, stunned, as he and the advisers rose and headed for the small private study nearby with a television. They turned it on and stood in a semicircle, silently, watching the videotaped replays of the disaster.

So began a day of tragedy that carried special poignancy for Reagan, who had spearheaded the idea of sending a schoolteacher, Christa McAuliffe, into space. Reagan had frequently celebrated space exploration as a symbol of America's superior technology and had planned to offer a soaring and confident vision of the nation's future last night in his State of the Union address to Congress.

Instead, the disaster threw the White House into a state of confusion and numbness. It was a day of such sudden shifts in mood and expectations that, for a while, the horror of the event and the upbeat rhetoric in the planned evening speech were mingled in a bizarre tableau in the White House Roosevelt Room.

There, the president's top assistants continued discussing the State of the Union message with television correspondents while, in the background, the scene of the exploding space shuttle was repeated on a large television screen that had been wheeled into the room.

Whenever a news bulletin about the disaster was broadcast, an aide signaled White House chief of staff Donald T. Regan, who stopped the discussion in the room to hear the latest developments.

An hour and twenty minutes after the explosion, the president came into the Roosevelt Room for what was supposed to be a sales pitch for the State of the Union address. Standing by a fireplace at the head of a long, oval table, Reagan immediately acknowledged to the television correspondents that there wasn't much sense in talking about the details of the planned speech that night.

But Reagan vowed to go ahead with the address despite the accident. "There could be no speech without mentioning this," he said. "But you can't stop governing the nation because of a tragedy of this kind. So, yes, one will continue."

The explosion was "a very traumatic experience," Reagan said, relating how he had watched it in the nearby study with Vice President Bush and national security affairs adviser John M. Poindexter, among others.

But Reagan said the lesson for the many school-children who had a special interest in the space mission was that "life does go on" and "you don't back up and quit some worthwhile endeavor because of tragedy."

According to White House officials, when Reagan spoke in the Roosevelt Room, he was still reluctant to accept the fact that all seven of the crew members had been killed, and this is why he thought the State of the Union might go ahead.

A television correspondent who was present in the Roosevelt Room observed that Reagan seemed determined to go ahead with the speech to make his point that one should not quit in the face of adversity.

Soon after the president finished talking, the network television correspondents went out to the White House lawn and broadcast reports that Reagan was planning to go ahead and deliver the State of the Union.

On Capitol Hill, House Minority Leader Robert H. Michel (R-Ill.) was watching television when these reports were aired. Michel immediately picked up the telephone and called the White House, according to an aide. His advice: postpone the speech.

White House aides Dennis Thomas and M.B. Oglesby were getting similar advice from other members. They told staff chief Regan that there appeared to be a consensus the State of the Union should be scrubbed. Officials said Regan then called House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and Senate Majority Leader Robert J. Dole (R-Kan.), who also urged that Reagan postpone the speech.

At that point, the president decided not to make the address. But there was still confusion about it.

Edward P. Djerejian, a deputy White House press secretary, wandered into the White House press room and was asked about the announcement on Capitol Hill, just moments before, that the speech was off. Djerejian said he would "reserve" comment on that and said White House spokesman Larry Speakes would make an announcement shortly. Reporters raced to their phones, believing that Djerejian had confirmed that the speech was off, but he shouted in vain, "No decision has been made!"

Five minutes later, Speakes announced that the State of the Union was postponed until next Tuesday. He said Bush was being sent to Cape Canaveral to carry Reagan's condolences to the families of the Challenger crew and to discuss the accident with officials of the National Aeronautics and Space Administration.

At times of crisis, the White House has far-flung facilities to gather information for the president, but yesterday was different. Television became the common denominator for everyone from Reagan to the harried workers in the White House press office. Even in the Situation Room, the basement communications center for the president, the main source of information in the first hours after the crisis were the network television reports.

The videotaped replays of the accident ran over and over in almost every room of the building, including one office with a scale model of the space shuttle perched on a nearby shelf.

Nancy Reagan was alone in the Executive Mansion when her press secretary, Elaine Crispen, telephoned with news of the disaster, but Mrs. Reagan had already turned on the television. "Oh my God, no," she said, according to Crispen. Mrs. Reagan "was in total shock" and "frozen to the set," saying "you pray for any thread of hope," she added.

White House officials said Mrs. Reagan also had influenced the president to postpone the State of the Union. The president then turned his attention to a draft of his brief, nationally televised statement on the accident, written by speechwriter Margaret (Peggy) Noonan.

Earlier, speaking to the television correspondents in the Roosevelt Room, Reagan maintained his own kind of optimism in the face of tragedy. Repeatedly, he brushed off suggestions that the Challenger explosion could be a sign of defeat or failure by the crew and the people involved.

"Well, I'm not a scientist," he said. "I do have confidence in the people that have been running this program and this is the first in . . . 56 some flights that something of this kind has happened. I certainly want everything done that can be done to find out how this could have happened and to ensure against its happening again. But . . . I have to say that I'm sure the people that have to do with this program are determined to do that right now."

Nor, despite the anguish and grief, did Reagan have any regrets for sending teacher Christa McAuliffe into space. "The world is a hazardous place, always has been in pioneering," he said, "and we've always known that there are pioneers that give their lives out there on the frontier.